Welcome to BYC_ there are so many great forums and articles about coop building and setting things up for your soon to be chickens. My only advice is - do it bigger than you think you need. Spend the money to make it secure and beautiful. If you do not, and cut corners you will be adding on soon- or revising. On the OTHER hand, if you feel like it is all overwhelming and you can't figure out what to do -- then quit stressing and do what you have to do to get started - the chickens don't care and over time you can improve it .
Hello and welcome to BYC! Glad you joined.
Build your coop as an open air coop with a huge amount of ventilation. Have one entire wall open and covered with nothing but hardware cloth. Build it in deep shade.